This invention provides a multi-channel photomultiplier free of cross-talk by providing a means to eliminate the transfer of electrons from one channel to another. Single photomultipliers have the advantage that they don't exhibit cross-talk. For economic reasons, one would like to pack many photomultipliers together to form a multi-channel photomultiplier. One of the problems with a multi-channel photomultiplier is that concomitant with obtaining high density is the problem of cross-talk between the individual channels.
At present there are several available types of multianode photomultipliers on the market, notably from Hamamatsu Photonics and Philips Components.
One new low cross-talk photomultiplier available commercially is the Philips XP1723. This photomultiplier contains a combination of a fiber optic window with an optimized dynode chain design with almost independent parallel operation of its 64 readout channels. To improve inter-channel separation the readout elements, dynode pads, are separated by small regions of "dead areas". However, even this photomultiplier, when low light levels are of the order of 10 photoelectrons or less per pulse, has a "confusion factor" of the order of 10.sup.-3 to 10.sup.-2. "Confusion factor" is defined here as the fraction of events allocated to wrong readout elements or pads after the signals are processed in the associated electronics and in a data analyzing computer. It is suspected that in this photomultiplier the cross-talk takes place mostly at the input stage between the photocathode and the first dynode and then in the dynode chain because the amplification dynode channels are not closed. As a result of the crosstalk, a center of gravity technique must be used to determine position. More importantly, the cross-talk prevents the use of this photomultiplier in a parallel mode with all the channels running independently and all at high rates. Another problem exhibited by the Philips XP1723 is its rather low quantum efficiency additionally compounded by poor transmission characteristics of its input fiber optic window.
In principle, there are other photodetectors that can be used to read many individual light channels but they have disadvantages which make their use difficult. Avalanche photodiodes lack enough amplification in a linear mode of operation to detect fast low amplitude signals from fibers. Avalanche photodiodes are also rather unstable, very sensitive to temperature and radiation and exhibit much noise. When used in a very high-gain Geiger amplification mode, with the gain above 10.sup.7, their rate capability is severely limited and they are very sensitive to radiation.
A solid state photomultiplier, Visible Light Photon Counter (VLPC), is difficult to operate because it has to be kept at .apprxeq.7.degree. Kelvin, and this introduces additional problems of coupling fibers to sensors.
An older structure which uses a low pressure gas as an amplifying element has lifetime problems as a result of the gases being incompatible with bialkali or multialkali photocathodes and the obtained amplification factors are low.
Hybrid photomultipliers with solid state elements were proposed in the 1960s and were recently reintroduced. Amplification is obtained by accelerating in a strong, 10 to 15 kVolt, electric field photoelectrons released at the photocathode and using them to bombard silicon diode active targets. Typical effective amplification factors of 2000 to 4000 are obtained with this method. In addition to silicon diodes, several other active elements were proposed, such as silicon drift chambers, silicon pixel arrays and avalanche diodes. Avalanche diodes add an additional amplification stage increasing the overall gain factor to 10.sup.7. Multichannel devices are in the process of being developed. The two main problems with a hybrid multianode device with an electrostatic amplifier are that the open, proximity focused geometry is vulnerable to the photocathode damage by ion feedback and that the active silicon element is sensitive to radiation damage. As a result, a relatively short lifetime can be expected.
Microchannel-based photomultipliers have also been built with external photocathodes. The existing devices suffer from count-rate and lifetime problems and are expensive. They also have the cross-talk problem because of the transversal movement of photoelectrons before they impinge on the microchannel plate. In one of the proposed novel designs dealing with this cross-talk effect, fibers are directly coupled, with no external photocathode, to the etched individual channels in a heavy metal oxide glass layer. One major problem with this design is the difficulty of making such a fiber-to-channel coupling scheme work with plastic fibers. Plastic fibers, due to their high air diffusion constant, are not compatible with the high quality vacuum, below 10.sup.-6 Torr, which is necessary for channel electron multipliers to operate. Another major problem with this design is that the efficiency of detection is expected to be very low as a result of the deposited photocathode material in the first section of the channel amplifier not being able to accelerate the photoelectrons sufficiently before impinging on the channel wall surface and, therefore, the secondary emission process will not take place.
U.S. Pat. No. 4,999,540 describes a stackable-dynode multiplier which is used in a single anode device and therefore is substantially different from a layered discrete guided electron multiplier designed for no cross-talk readout of many channels.
U.S. Pat. No. 3,240,931 describes an array of channel electron multipliers, however they are positioned to receive particles from a charged particle beam. No application with the array coupled to a photocathode to form a multi-channel photomultiplier is discussed.
In U.S. Pat. No. 4,937,506 the problem of electron divergence in the photomultiplier tube related to the issue of cross-talk is addressed by using focusing grid electrodes between the photocathode and the dynodes and/or between adjacent mesh dynodes and/or between the last dynode and the anode. However, this design is not completely cross-talk free as a result of the barriers between channels not being 100% impenetrable to electrons.
There are, in principle, several sources of cross-talk in multi-channel photomultipliers:
(1) photon cross-talk at the input window due to a photon beam spot size and divergence and to scattering of arriving photons; this effect can be minimized by using a fiberoptic faceplate window or individual focusing elements such as small spherical lenses outside the photomultiplier window or built into the photomultiplier window; additionally the window can be made of many small optical windows mounted in a holding window frame;
(2) photoelectron cross-talk with photocathode electrons traveling too far transversally and starting electron avalanches far away from the production point and even falling into "wrong" channels in the devices with discrete dynode structures;
(3) dynode electron cross-talk due to transversal electron avalanche growth encompassing more than one anode readout element (pad); this effect is mostly seen in discrete dynode structures with no limiting barriers or walls such as mesh dynodes;
(4) electron cross-talk at the anode readout elements; electrons emerging from dynode amplification chains can additionally spread transversally and be collected on several neighboring channels, pads, strips or wires; and
(5) electronic cross-talk due to capacitive coupling and interchannel pickup, and also via the last common dynode.